The present invention relates to a gas sensing element incorporated in a gas sensor used for controlling an air-fuel ratio of the gas mixture introduced into a combustion chamber of an internal combustion engine.
In an automotive vehicle, a gas sensor is provided in an exhaust pipe of an internal combustion engine to control the air-fuel ratio of the gas mixture introduced into a combustion chamber of the engine.
For example, a practical gas sensing element incorporated in the gas sensor is required to detect a concentration of oxygen gas contained in the exhaust gas. The gas sensing element may be formed by a ZrO2 solid electrolytic body which is capable of generating an electromotive force in accordance with the oxygen concentration.
Japanese Patent No. HEI 2-15017 discloses a conventional gas sensing element whose output is responsive to an electromotive force.
According to this type of conventional oxygen concentration detector, a gas sensing element is provided at a distal end of this detector. The gas sensing element is configured into a cup-shaped multilayered structure comprising a reference gas sensing electrode, a solid electrolytic sintered body, a measured gas sensing electrode, and an electrode protective layer which are successively stacked in this order. A heater is accommodated in an inside chamber of the cup-shaped sensing element.
The electrode protective layer is a ceramic coating layer, or a double layer consisting of a ceramic coating layer and a xcex3-Al2O3 layer provided on this ceramic coating layer.
Exhaust gas reaches the measured gas sensing electrode through the ceramic coating layer or the double layer of the ceramic coating layer and the xcex3-Al2O3 layer.
Recently enacted regulations relating to exhaust gas emissions have forced automotive manufacturers to develop automotive engines having the capability of precisely controlling the combustion. To realize this, it is essentially important to provide an excellent gas sensor having sensing properties stable under severe exhaust gas temperatures and durable for a long time.
One of the main factors causing change in sensing properties is deterioration of the gas sensing element subjected to poisonous substances such as Pb, S etc. contained in gasoline, additives K, Na, P, Ca, Zn etc. involved in gasoline or oil and the Si component contained in a seal member of the internal combustion engine.
U.S. Pat. No. 5,766,434 (corresponding to JP10-221296) proposes a gas sensing element comprising a measured gas sensing electrode covered by a protective layer including coarse particles and fine particles structurally arranged in such a manner that interparticle cavities formed between the coarse particles are filled with the fine particles.
This conventional technique intends to adequately set a porosity and a pore diameter of the protective layer by filling the coarse interparticle cavities with the fine particles. Anti-poisoning properties can be improved. Obtained gas choking effect will cause unburnt components in the exhaust gas to react with residual oxygen in an equilibrium condition. The waveform of a sensor output will be sharpened.
The above-described prior art effectively prevents blinding or plugging of the protective layer subjected to crystalline or glass substances formed by gasoline or oil compounds. Accordingly, the gas sensing element can be effectively protected from poisonous substances.
However, the above-described prior art cannot bring satisfactory effects against poisonous substances of gas phase, such as Si components, contained in the seal member or the like of the internal combustion engine.
Poisoning mechanism of Si components is as follows.
Si components of gas phase residing in the vicinity of a gas sensing element penetrate the protective layer and subsequently reach the measured gas sensing electrode. The measured gas sensing electrode, when covered by Si components, loses its activity. As a result, Si components deteriorate the sensing properties of the gas sensing element.
In view of the above-described problems, the present invention has an object to prevent poisonous substances, such as gaseous Si components, from reaching the measured gas sensing electrode. In other words, an object of the present invention is to prevent the measured gas sensing electrode from being covered by poisonous substances. Thus, the present invention provides a gas sensing element capable of maintaining a stable sensor output for a long term.
To accomplish the above and other related objects, the present invention provides a first gas sensing element comprising a solid electrolytic body, a reference gas sensing electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, and a measured gas sensing electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. A ceramic porous protective layer is provided on a surface of the measured gas sensing electrode. The protective layer comprises coarse particles and fine particles structurally arranged in such a manner that inter particle cavities formed between the coarse particles are filled with the fine particles. And, at least either of the coarse particles and the fine particles contain at least one selected from the group consisting of xcex3-Al203, xcex8-Al203, xcex4-Al203 and solid solution having the same crystal structure as those of xcex3-Al203, xcex8-Al203, and xcex4-Al203.
The first gas sensing element of the present invention is characterized in that at least either of the coarse particles and the fine particles constituting the protective layer contain at least one selected from the group consisting of xcex3-Al2O3, xcex8-Al2O3, xcex4-Al2O3 and solid solution having the same crystal structure as those of xcex3-Al2O3, xcex8-Al2O3, xcex4-Al2O3. More specifically, the solid solution is an alumina solid solution having the crystal structure of either a tetragonal system or a monoclinic system which are the same crystal structures as those of xcex3-Al2O3, xcex8-Al2O3, xcex4-Al2O3.
The coarse particles and the fine particles can be made of the same material or can be made of different materials.
For example, practical examples of the solid solution are xcex3-(Al1-xLax)2O3, xcex8-(Al1-xLax)2O3, and xcex4-(Al1-xLax)2O3.
The protective layer covers the entire surface of the measured gas sensing electrode. It is also possible to provide the protective layer so as to cover the surface of the solid electrolytic body together with the measured gas sensing electrode.
Furthermore, it is possible to provide another layer between the protective layer and the measured gas sensing electrode (refer to FIG. 1).
Effects of the first gas sensing element of the present invention will be explained hereinafter.
The protective layer of the first gas sensing element consists of coarse particles and fine particles. Accordingly, interparticle cavities formed between the coarse particles are filled with the fine particles. It becomes possible to sufficiently reduce a porosity and a pore diameter of the protective layer.
Furthermore, in a process of forming the protective layer, the coarse particles cooperatively constitute bridges which effectively prevent generation of cracks in the protective layer. Thus, the present invention makes it possible to easily obtain a protective layer having a satisfactory thickness.
The coarse particles and fine particles constituting the protective layer contain a material having a large specific surface, such as xcex3-Al2O3, xcex8-Al2O3, and xcex4-Al2O3.
Accordingly, it becomes possible to enlarge a substantial contact area of the protective layer to be exposed to poisonous gaseous substances. Thus, the protective layer surely traps the poisonous substances.
In this manner, the present invention provides a protective layer which can surely trap incoming poisonous gaseous substances, thereby surely preventing the poisonous substances from reaching the measured gas sensing electrode.
Hence, it becomes possible to maintain a stable sensor output for a long time.
As described above, the present invention can provide a gas sensing element which is capable of preventing poisonous gaseous substances, such as Si components, from reaching the measured gas sensing electrode, and is accordingly capable of preventing the measured gas sensing electrode from being covered by the poisonous gaseous substances, and therefore capable of maintaining a stable sensor output for a long time.
Thus, the present invention provides a gas sensing element preferably installable in an exhaust system of an internal combustion engine.
It is preferable that the alumina material constituting the protective layer, such as xcex3-Al203, xcex8-Al203, and xcex4Al203 or solid solution having the same crystal structure as those of xcex3-Al203, xcex8-Al203, and xcex4-Al203 has a specific surface equal to or larger than 50 m2/g. The effect of this invention can thereby be enhanced.
If the specific surface of the alumina material or relevant solid solution is smaller than 50 m2/g, the ability of trapping poisonous materials of individual particles will be dissatisfactory and therefore the sensor will suffer from the shortage of poisonous substance trapping capability.
It is preferable that the protective layer has a thickness equal to or larger than 50 xcexcm. This makes sure that the poisonous substances can be sufficiently brought into contact with the particles of xcex3-Al2O3 or the like. Thus, the sensor performance can be improved so as not to be adversely influenced by poisonous substances.
If the thickness of the protective layer is smaller than 50 xcexcm, the poisonous substances will not be sufficiently trapped by the particles. Hence, the sensor capability of trapping poisonous substances will be worsened.
Application of the present invention is not limited to a cup-shaped gas sensing element shown in FIG. 1. Therefore, the present invention can be applied to other types of gas sensing elements, such as a multilayered planar gas sensing element.
Furthermore, the gas sensor of the present invention can be used as an oxygen sensor or an air-fuel ratio sensor installed in an automotive internal combustion engine, or as a NOx sensor, a CO sensor, a HC sensor of a multilayered type.
According to the present invention, it is preferable that the protective layer contains a noble metallic catalyst.
In general, noble metallic catalysts equilibrate unburnt gas and accordingly have a function of stabilizing a sensor output. In other words, the noble metallic catalyst assists or improves the ability of trapping poisonous gaseous substances in the protective layer. Thus, the effect of the present invention can be further enhanced.
Furthermore, as described later with reference to FIG. 5, it is preferable that the noble metallic catalyst is disposed in a discrete manner around coarse particles or between fine particles.
For example, the noble metallic catalyst is Pt, Rh, Pd etc.
Furthermore, it is preferable that an amount of the noble metal contained in the protective layer is in a range from 0.1% to 5%.
This ensures the gas equilibrating function of the noble metal, thereby further enhancing the sensor capability of trapping poisonous substances.
If the content of the noble metal is less than 0.1%, the effect of the present invention will not be obtained. If the content of the noble metal exceeds 5%, an adsorbing time of the noble metal required to adsorb the gas components will be fairly extended in a process of equilibrating the unburnt gas and the residual oxygen. This will worsen the sensor response, even through the poisonous substance trapping capability may be adequately maintained.
Furthermore, according to the present invention, it is preferable that the protective layer satisfies a relationship that a ratio RB/RA is equal to or larger than 3, where RA represents an average particle diameter of the fine particles while RB represents an average particle diameter of the coarse particles.
When the ratio RB/RA is equal to or larger than 3, the interparticle cavities formed between the coarse particles are efficiently filled with the fine particles. The porosity and the pore diameter of the protective layer can be reduced adequately. The poisonous substance trapping capability can be enhanced.
If the ratio RB/RA is smaller than 3, it will become difficult to sufficiently fill the interparticle cavities formed between the coarse particles with the fine particles. The poisonous substance trapping capability will be deteriorated.
Furthermore, it is preferable that an upper limit of RA/RB is set to 150 in view of stability of a slurry used to form the protective layer.
Furthermore, according to the present invention, it is preferable that an average particle diameter of the fine particles is in a range from 0.1 xcexcm to 5 xcexcm and an average particle diameter of the coarse particles is in a range from 0.3 xcexcm to 50 xcexcm. This is effective to obtain a protective layer having an excellent poisonous substance trapping capability.
If the average particle diameter of the fine particles is less than 0.1 xcexcm, cracks will be caused in the process of forming the protective layer. The poisonous substance trapping effect will be lowered. If the average particle diameter of the fine particles exceeds 5 xcexcm, dispersion of slurry particles used to form the protective layer will be no good. Thus, it will be difficult to obtain a homogeneous protective layer. The poisonous substance trapping effect will be lowered.
Similarly, if the average particle diameter of the coarse particles is less than 0.3 xcexcm, cracks will be caused in the process of forming the protective layer. The poisonous substance trapping effect will be lowered. If the average particle diameter of the coarse particles exceeds 50 xcexcm, dispersion of slurry particles used to form the protective layer will be no good. Thus, it will be difficult to obtain a homogeneous protective layer. The poisonous substance trapping effect will be lowered.
Next, according to the present invention, it is preferable that the protective layer satisfies a relationship that a weight ratio WA/W is equal to or larger than 20, wherein W is a sum of WA and WB (i.e., W=WA+WB), WA represents an amount of the fine particles contained in the protective layer, and WB represents an amount of the coarse particles contained in the protective layer.
When the weight ratio WA/W is equal to or larger than 20, the interparticle cavities formed between the coarse particles are efficiently filled with the fine particles. The porosity and the pore diameter of the protective layer can be reduced adequately. The poisonous substance trapping capability can be enhanced.
If the weight ratio WA/W is excessively large, the efficiency of filling the coarse interparticle cavities with the fine particles will be worsened. A preferable upper limit of the weight ratio WA/W is 50.
Furthermore, the present invention provides a second gas sensing element comprising a solid electrolytic body, a reference gas sensing electrode provided on a surface of the solid electrolytic body so as to be exposed to a reference gas, a measured gas sensing electrode provided on another surface of the solid electrolytic body so as to be exposed to a measured gas. A ceramic porous protective layer is provided on a surface of the measured gas sensing electrode. The protective layer comprises coarse particles and fine particles arranged in such a manner that interparticle cavities formed between the coarse particles are filled with the fine particles. And, at least either of the coarse particles and the fine particles have a specific surface equal to or larger than 50 m2/g.
The protective layer of the second gas sensing element of the present invention consists of coarse particles and fine particles. Accordingly, the interparticle cavities formed between the coarse particles are filled with the fine particles. It becomes possible to sufficiently reduce a porosity and a pore diameter of the protective layer.
Furthermore, in a process of forming the protective layer, the coarse particles cooperatively constitute bridges which effectively prevent generation of cracks in the protective layer. Thus, the present invention makes it possible to easily obtain a protective layer having a satisfactory thickness.
The coarse particles and fine particles constituting the protective layer contain a material having a large specific surface.
Accordingly, it becomes possible to enlarge a substantial contact area of the protective layer to be exposed to poisonous substances of gas phase. Thus, the protective layer surely traps the poisonous substances.
In this manner, the present invention provides a protective layer which can surely trap incoming poisonous gaseous substances, thereby surely preventing the poisonous substances from reaching the measured gas sensing electrode.
Hence, it becomes possible to maintain a stable sensor output for a long time.
If the specific surface is less than 50 m2/g, the ability of trapping poisonous materials of individual particles will be dissatisfactory and therefore the sensor will suffer from the shortage of poisonous substance trapping capability.
As described above, the present invention can provide a gas sensing element which is capable of preventing poisonous gaseous substances, such as Si components, from reaching the measured gas sensing electrode, and accordingly capable of preventing the measured gas sensing electrode from being covered by the poisonous substances of gas phase, and therefore capable of maintaining a stable sensor output for a long time.